Wobble piston pump with carbon graphite cylinder

ABSTRACT

A two-cylinder axial displacement wobble piston air compressor/vacuum pump ( 10 ) has a pair of open-ended cylinder sleeves ( 36 ) lining the pump chambers ( 34 ) of the housing ( 16 ).The cylinder sleeves are made of a pressed powder carbon graphite material that is impregnated with a phenolic resin. The wobble pistons ( 18 ) include a piston cup ( 48 ) made of a PTFE material and having a diameter sized to abut the bore of the cylinder throughout the compression and suction strokes of the piston.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. provisional application Ser. No. 60/311,884, filed Aug. 13, 2001.

STATEMENT OF FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

This invention relates to pumps and in particular to pumps with improved piston-cylinder arrangements.

Pistons pumps have one or more pistons is eccentrically mounted on a rotatable shaft to reciprocate the pistons in a cylindrical pump chamber. As the pistons reciprocate, a suitable valve assembly, having intake and exhaust valves which alternately open and close during the suction and pump strokes, moves air into and out of the pump chamber.

As the pistons reciprocate within the cylinders, the pistons (or circumferential piston rings) contact the inner walls of the cylinders. Friction generates heat, which can degrade the cylinders and/or the pistons (or piston rings) over time. The thermal effects can also cause dimensional changes in the pistons or cylinders which are designed to interact within tight tolerances. Various techniques have been developed to reduce these adverse effects.

For example, U.S. Pat. No. 1,467,255 discloses a low friction piston composed of monolithic carbon graphite. U.S. Pat. No. 2,792,265 discloses a piston-cylinder arrangement in which the cylinder includes cooling fins, and the piston is constructed with a cast iron core supporting a carrying ring, sealing rings and a sleeve made of self-lubricating material, such as carbon. U.S. Pat. No. 2,860,935 discloses a complex piston arrangement with adjustable rings.

These techniques are not applicable to air pumps having axial displacement wobble-type pistons that pivot as they reciprocate. Wobble pistons typically have a thin-cup element mounted to the head of the piston. The outer periphery of the cup slides against the inner diameter of the cylinder. As the piston wobbles, the periphery of the cup flexes to accommodate the pivot of the piston and press tightly against the cylinder to provide a tight seal.

Thus, an improved technique is needed for use with wobble-type pistons.

SUMMARY OF THE INVENTION

The invention is a pump providing an improved wobble piston and cylinder arrangement.

Specifically, the invention provides a pump with a housing containing a rotatable shaft extending into a pump chamber having a carbon graphite cylinder through which travels a wobble piston eccentrically mounted at one end to the shaft and having a head at an opposite end that is pivotal with respect to a cylinder axis.

In one form, the cylinder is an open-ended cylinder sleeve separate from the crankcase and lining the pump chamber. The cylinder sleeve is preferably a pressed powder carbon graphite material that is impregnated with a thermoset resin, such as a suitable phenolic resin.

In another form, the invention provides a pump having a housing defining a pump chamber and containing a rotatable shaft extending into the pump chamber. The pump chamber is lined with a carbon graphite cylinders A wobble piston is mounted to the shaft so as to reciprocate within the cylinder and pivot with respect to a cylinder axis. The piston includes a piston cup fixedly mounted to a head of the piston opposite its connection to the shaft. The piston cup has a periphery larger than the piston head so as to abut an inner diameter of the cylinder throughout compression and suction strokes of the piston. The piston cup is preferably a PTFE composition.

A multi-cylinder pump, such as a two or four cylinder pump, can also be provided with the crankcase defining multiple pump chambers, each having an associated wobble piston and carbon graphite cylinder.

The carbon graphite cylinder can be machined to precise dimensions for interface with the contacting portion of the piston. The graphite material has lower thermal expansion and friction coefficients compared to standard aluminum cylinders, thereby reducing wear and making the cylinder more dimensionally stable. The cylinder also reduces the operating temperatures of the cup, which reduces fatigue and creep that could otherwise lead to permanent deformation, thereby prolonging the life of the cup. The phenolic resin makes the cylinder air tight by filling any voids that may have been present in the graphite.

These and other advantages of the invention will be apparent from the detailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a two-cylinder axial displacement wobble piston vacuum pump according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With continuing reference to FIG. 1, the present invention provides an axial displacement wobble piston pump preferably used to provide forced air movement in vacuum pumps and air compressors of various sizes and capacities, however, the present invention could be used for other pumping applications and media. A dual-cylinder vacuum pump 10 includes a motor 12, such as an AC electric motor, driving a pair of identical pump units 14. Each pump unit 14 includes a housing 16 containing a wobble piston 18 mounted to a shaft 20 which is rotated by the motor 12. The vacuum pump 10 includes two pump units 14, however for simplicity, only one of the pump units 14 will be described in detail, although both will be numbered as applicable.

Each housing 16 has a motor end 24 and an open access end 26 covered by an end cap (not shown) suitably mounted to the housing 16. The housing 16 defines a generally cylindrical crankcase concentric with the shaft 20 and separated from the motor end 24 by a partition supporting a bearing journaling the shaft 20 to the housing 16. Preferably, the motor rotor is fixed to the shaft 20 and the stator shell is press-fit onto the motor end 24 of the housing 16. A fan 28 is mounted on the end of the shaft 20 adjacent the access end 26 and ventilation slots are formed in the end cap for drawing cooling air into the housing 16.

The housing 16 also defines a neck 30 extending upwardly perpendicular to the shaft 20. The neck 30 has an open throat 32 in common with the access end 26 of the housing 16. The neck 30 also defines a cylindrical pump chamber 34 with a flat ledge (not shown) around much of its circumference other than at the open throat 32 to provide a flat surface for supporting a cylinder sleeve 36 inside the pump chamber 34 closing off the open neck 32.

The cylinder sleeve 36 is an open ended cylinder separate from the housing 16 with an outer diameter sized to fit inside the pump chamber 34 and an inner diameter machined to a bore diameter required for the wobble piston 18. The cylinder sleeve 36 is made entirely of a suitable grade of carbon graphite. The carbon graphite selected should be sufficiently rigid while having as low of friction and thermal expansion coefficients as possible. Regardless of the grade selected, however, the carbon graphite cylinder sleeve 36 will have lower friction and thermal expansion coefficients than conventional aluminum cylinders. Preferably, the cylinder sleeve 36 is formed of graphite rods machined to the appropriate outer and bore diameters. It should be noted, however, that carbon graphite sheets and blocks could also be used. The stock carbon graphite rods are preferably made of powdered carbon graphite that is pressed and impregnated with a thermoset resin, such as a phenolic resin, as known in the art. The phenolic resin fills any voids that may otherwise have existed in the carbon graphite to provide a non-porous, air tight material.

Within the cylinder sleeve 36 is the wobble piston 18. The wobble piston 18 includes a connecting rod 38 preferably having a pressed bearing 40 for receiving an eccentric element 42 that mounts onto the shaft 20 by a suitable means, such as a set screw connection. The eccentric element 42 has a bore along the shaft axis that is eccentric to its outer diameter so that the piston 18 reciprocates and pivots a few degrees on each side of a vertical cylinder axis 43 so that it “wobbles” within the cylinder sleeve 36 as the shaft 20 rotates. A counter-weight 44 is also provided.

At the opposite end of the connecting rod 38 is the piston head having a circular flange 46 that supports a piston cup 48 with a cup shim 50 beneath it and a cup retainer 52 above it, all of which is secured to the flange 46 via a suitable fastener 54 disposed through the center of the disk-shaped components and threaded into the connecting rod 38. The piston cup 48 has a cupped center opening upward and a periphery extending radially outward past the flange 46 and retainer 52 so as to abut the inner bore of the cylinder sleeve 36. The piston cup 48 is preferably a PTFE material that is sufficiently stiff but is flexible relative to the rest of the wobble piston 18 and the cylinder sleeve 36 so that the periphery of the piston cup 48 will flex as needed to accommodate the pivoting of the piston head.

Flow of air into and out of the pump chamber 34 is controlled by a valve head assembly having a valve plate 56 with inlet 58 and exhaust 60 valves that move in response to air pressure, as known in the art, and which are in communication with inlet 62 and exhaust 64 chambers in a valve head 66. One of the inlet 62 chambers has a fitting 68 for attaching air lines and the other is connected to a connector tube 70 coupled to the exhaust chamber of the valve head with the intake fitting 68. A muffler 72 is connected to the exhaust chamber 64 of the valve head 66 without the intake fitting 68. The valve head 66 and valve plate 56 are fastened to the housing 16 with threaded fasteners 74. The valve plate 56 has a circular groove (at its bottom side) in which fits the top rim of the cylinder sleeve 36 having a reduced dimension. An o-ring 76 fits around the cylinder sleeve 36 in abutment with the underside of the valve plate 56. A suitable sealing ring or gasket 78 is disposed between the valve plate 56 and the valve head 66 to prevent air leakage.

In a preferred embodiment, the housing 16 is made either of an aluminum alloy. The piston connecting rod 38 and cup retainer 52 are preferably made of aluminum alloy. The valve head 66 and valve plate 56 are aluminum. And, as mentioned, the piston cup 54 is PTFE and the cylinder sleeve is carbon graphite.

As the motor 12 is operated the eccentric elements 42 cause the piston heads to reciprocate back and forth in the cylinder sleeves 36 with a wobble motion. The is pistons 18 are mounted to be 180 degrees out of phase with each other so that the piston cups 48 alternately draw air in through their respective inlet valves and into the pump chambers 34 on the downstroke (compression stroke) and force air through the exhaust valves to the valve heads 66 on the upstroke (exhaust stroke). The valves of the valve head 56 alternately open and close in response to the reciprocating pistons 18 to allow air to enter and exit the right-hand pump unit and pass into the left-hand pump unit via the connector tube 70 where it is exhausted through the muffler 72.

It should be noted that pump is described herein as a vacuum pump in which ambient air is high pressure (atmospheric), however, the same device could operate as an air compressor so that the ambient air is the lower pressure. In this case, as the pump chamber is pressurized during the compression stroke, the piston cup will flex under pressure and be pressed against the inner bore of the cylinder sleeve, thereby making a tight seal preventing air from leaking around the wobble piston.

The invention thus provides a pump with an improved wobble piston and cylinder arrangement. The carbon graphite cylinder can be machined to precise dimensions for interface with the piston cup. The graphite material has lower thermal and friction coefficients compared to standard aluminum cylinders, thereby reducing wear and making the cylinder more dimensionally stable. The cylinder also reduces the operating temperatures of the piston cup, which reduces fatigue and creep that could otherwise lead to permanent deformation, thereby prolonging the life of the piston cup.

A preferred embodiment of the invention has been described in detail above. However, modifications and variations to the preferred embodiment will be apparent to those skilled in the art, which will be within the spirit and scope of the invention. Therefore, the invention should not be limited to the described embodiment. To ascertain the full scope of the invention, the following claims should be referenced. 

1. In a pump having a housing containing a rotatable shaft extending into a pump chamber having a cylinder through which travels a wobble piston eccentrically mounted at one end to the shaft and having a head at an opposite end that is pivotal with respect to a cylinder axis, the improvement wherein the cylinder is a carbon graphite composition.
 2. The improvement of claim 1, wherein the cylinder is an open-ended cylindrical sleeve separate from the housing lining the inside of the pump chamber.
 3. The improvement of claim 2, wherein the cylinder is a pressed carbon graphite powder.
 4. The improvement of claim 3, wherein the carbon graphite powder is impregnated with a thermoset resin.
 5. The improvement of claim 4, wherein the thermoset resin is a phenolic.
 6. The improvement of claim 1, wherein the piston head includes a piston cup having a periphery with a diameter larger than the piston head sized to abut an inner diameter of the cylinder throughout compression and suction strokes.
 7. The improvement of claim 1, wherein the housing includes multiple pump chambers, each having an associated wobble piston and graphite cylinder.
 8. A pump, comprising: a housing defining a pump chamber and housing a rotatable shaft extending into the pump chamber; a carbon graphite cylinder lining the inside of the pump chamber; and a wobble piston mounted to the shaft so as to reciprocate within the cylinder and pivot, with respect to a cylinder axis, wherein the piston includes a piston cup fixedly mounted to a head of the piston opposite its connection to the shaft and having a periphery with a diameter sized to abut an inner diameter of the cylinder throughout compression and suction strokes of the piston.
 9. The pump of claim 8, wherein the piston cup is a PTFE composition.
 10. The pump of claim 9, wherein the cylinder is a pressed carbon graphite powder.
 11. The pump of claim 10, wherein the carbon graphite powder is impregnated with a phenolic resin.
 12. The pump of claim 8, wherein the housing includes multiple pump chambers, each having an associated wobble piston and carbon graphite cylinder. 